The present invention relates a method of preserving a food product, wherein the food product is forced through a treatment system having an entrance and an exit while a pressure difference of at least 10 MPa is substantially maintained between the entrance and the exit, wherein the product is subjected to a varying electric and/or magnetic field, and to an apparatus for preserving food products, comprising at least one treatment system having an entrance and an exit, a high-pressure generating means for forcing a product through the treatment system, while substantially maintaining a pressure difference of at least 10 MPa between the entrance and the exit, and means for generating a varying electric and/or magnetic field and for subjecting the product to the varying electric and/or magnetic field. "Entrance" and "exit" are herein to be understood as to describe the part where at least part of the treatment occurs. Prior to the entrance, there will be pressurising means, and prior to the pressurising means there will be a storage of food product which is to be preserved. Hence, the foodproduct to be preserved will go from storage, pressurising means, entrance, a phase of high pressure, pressure reduction, to an exit.
Such a method and apparatus are disclosed in International patent application WO 97/43914. Ultra-high pressure (UHP) preservation is a technique which allows micro-organisms in food products to be destroyed, or at least inactivated, without substantially increasing the temperature of the product. Thus, the disadvantages of heat treatments such as pasteurisation and sterilisation, often affecting the quality (taste, nutrients) of the product, are avoided. The pressures involved in UHP-preservation are extremely high, typically several hundred MPa. The method described allows a continuous process which especially in the food industry is particularly advantageous.
In the known apparatus, the treatment system includes one treatment chamber having a load impedance, wherein the means for generating the varying field has a characteristic impedance matched to the load impedance in order to obtain an optimal energy transfer from the generating means to the treatment chamber. However, an unwanted effect of the varying field is that the food product is also heated up during the treatment. Under normal operation, the heating of the product can reach 30.degree. C. or more. Heating the product causes a decrease of the load impedance of the treatment chamber and this impedance change results in a temperature dependent mismatch of the load impedance and the characteristic impedance of the generating and subjecting means. This unwanted effect results in an energy transfer between the generating means and the treatment chamber which is temperature dependent.